This invention relates to semiconductor light emitting devices and fabricating methods therefore, and more particularly to packaging and packaging methods for semiconductor light emitting devices.
Semiconductor light emitting devices are known for use in a variety of light source applications. For example, light emitting diodes (or LEDs) are well known solid state electronic devices capable of generating light upon application of a sufficient voltage. Light emitting diodes generally comprise a p-n junction formed in an epitaxial layer deposited on a substrate, such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED generally depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that comprise the light generation region of the device.
Commonly, an LED includes an n-type substrate, an n-type epitaxial region formed on the n-type substrate and a p-type epitaxial region formed on the n-type epitaxial region. In order to facilitate the application of a voltage to the device, an anode ohmic contact may be formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode ohmic contact may be formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer). The ohmic contacts, thus, may provide contact nodes for connecting the LED to an electronic circuit.
As illustrated in FIG. 1, a conventional LED 70 may be packaged in a standard package 72 that may include a conductive/reflective mounting cup 73 that, in turn, may be connected to a cathode/anode lead 75A. The LED chip 70 is typically mounted in the cup with a silver epoxy. The anode/cathode of the LED chip 70 may be wirebonded to an anode/cathode lead 75B. The entire package may then be encapsulated in, for example, a transparent epoxy 78. For white-emitting LED chips, the encapsulant 78 may include a wavelength converter material, such as a wavelength-converting phosphor. In a typical white LED application, some of a blue light emitted from the chip 70 stimulates the wavelength conversion material to emit longer wavelength light, such as yellow light. The “unconverted” blue light from the chip combines with the longer wavelength light emitted from the phosphor to synthesize white light, which may be emitted from the package.
Flip-chip mounting of LEDs involves mounting the LED onto the submount substrate side up. Light may then be extracted and emitted through the transparent substrate. Flip chip mounting may be a desirable technique for mounting SiC-based LEDs. Because SiC generally has a higher index of refraction than GaN, light generated in the light emitting region may not internally reflect (i.e. reflect back into the GaN-based layers) at the GaN/SiC interface. Flip chip mounting of SiC-based LEDs may offer improved light extraction when employing certain chip-shaping techniques known in the art. Flip chip packaging of SiC LEDs may have other benefits as well, such as improved heat extraction/dissipation, which may be desirable depending on the particular application for the chip.
Some problems commonly encountered with conventional white LED packages relate to uniformity of emission and the size of the optical image generated by the package. Conventional packages tend to have a large optical image size, which is typically a function of the size of the metallic mounting cup. Moreover, because the wavelength conversion material may be distributed over a large area compared to the size of the LED chip and because the exact distribution of wavelength conversion material within the coating may be difficult to control, the uniformity and reproducibility of the light output may suffer.